The pressure-sensing and electrical switch components of pressure-operated switches are generally built into the instrument and, when supplied by the manufacturer, are mounted on an instrument panel and connected to the pressure and electric lines at the instrument panel by shutoff valves, T's, junctions and the like which require the services of licensed plumbing and electrical workers. As thus connected, in the event that an instrument must be replaced, repaired, calibrated or tested, it must be removed which, again, requires the services of licensed plumbing and electrical workers, which is expensive and represents a loss of service time. Moreover, the necessity for employing shutoff valves, junctions and the like at the mounting panel makes it difficult to bank the instruments compactly to reduce the space required for instrumentation to a minimum. It is the purpose of this invention to provide a pressure switch wherein the pressure-sensing and electrical components may be removed as a self-contained unit from the instrument for replacement, repair, calibration, testing and the like without requiring dismounting of the instrument as a whole from the mounting panel and, hence, without the need for shutoff valves, T's, junctions and the like. The provision of a self-contained unit has the further advantages that, when removed for repair, testing, calibration and the like, it may be replaced by a standby unit without loss of time and by shop personnel while the malfunctioning unit is being repaired or tested; that it is possible for the manufacturer to construct instrument panels with instruments already mounted thereto with the exception of the sensing and electrical units, thereby providing a substantial saving in manufacturing costs and initial installation cost; making possible more uniform arrangement of the instruments on the panel; providing for expansion of services without entailing the added cost of the sensing units by including in the instrument panel as constructed one or more empty instruments which may be on the spot completed by adding a sensing unit thereto; and a unit which can be used independently of the instrument for comparison purposes. Other advantages of construction obtained are improved repeatability and sensitivity over a range of 3 to 15 psi; on and off differentials of 1.624 inches wc with a repeatability of a set point of plus or minus 1/2 percent fs; a tamperproof resistance scale adjustment and the option of electrical continuity when the sensing component is removed.